A method of treating sewage sludge by heating and/or evaporating and/or pasteurizing and/or otherwise chemically treating material comprising sludge and any added ingredients. The method includes: (a) providing a drum (D) (b) delivering (40, 41, 42) the material to the drum (c) tumbling (126) the material within the drum (d) heating the material in the drum (e) drawing off moisture gases being evaporated from the material in the drum (f) discharging (135) the material from the drum via a discharge outlet (134) and (g) ascertaining the rate of evaporation of moisture from the drum. Several techniques are provided for ascertaining the rate of evaporation of moisture from the drum. The method may also include: (h) adding material into the drum in amounts responsive to the rate of evaporation of moisture from the drum.
